The present invention relates generally to diagnostic imaging and, more particularly, to a method and apparatus to optimize dose efficiency by dynamically filtering radiation emitted toward the subject during radiographic imaging in a manner tailored to the position and/or shape of the subject to be imaged.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis and subsequent image reconstruction.
There is an increasing desire to reduce radiation expose to a patient during radiographic data acquisition. It is generally well known that significant radiation or “dose” reduction may be achieved by using an attenuation filter to shape the intensity profile of an x-ray beam. Surface dose reductions may be as much as 50% using an attenuation filter. It is also generally known that radiation exposure for data acquisition from different anatomical regions of a patient may be optimized by using specifically shaped attenuation filters tailored to the anatomical region-of-interest (ROI). For example, scanning of the head or a small region of a patient may be optimized using a filter shape that is significantly different than a filter used during data acquisition from the heart. Therefore, it is desirable to have an imaging system with a large number of attenuation filter shapes available to best fit each patient and/or various anatomical ROIs. However, fashioning an imaging system with a sufficient number of attenuation filters to accommodate the numerous patient sizes and shapes that may be encountered can be impractical given the variances in a possible population. Additionally, manufacturing an imaging system with a multitude of attenuation filters would increase the overall manufacturing cost of the imaging system.
Further, for optimum dose efficiency, i.e. best image quality at the lowest possible dose, the attenuation profile created by the attenuation filter should be particular to the patient. That is, it is desirable and preferred that when selecting a pre-patient or attenuation filter that it be adjusted according to the particulars of the patient, such as the patient's size, shape, and relative position in the bore of the scanner, be taken into account. By taking these and other particulars into consideration, radiation exposure can be optimized for the patient and the scan session.
Known CT scanners use both an attenuation filter and dynamic current modulation to shape the intensity of the x-ray beam incident to the patient. To reduce radiation exposure, the attenuation filer is typically configured to minimize x-ray exposure to edges of the patient where path lengths are shorter and noise in the projection data has a less degrading impact on overall image quality. Accordingly, one such implementation of the attenuation filter is the bowtie filter, which, as a function of form, increases attenuation of x-ray intensity incident upon of the peripheral of the imaging subject. However, improper patient centering and/or bowtie filter selection can significantly degrade image quality and dose efficiency because x-ray attenuation is misapplied to the particulars of the subject. The bowtie filter is aligned with a point of maximum radiation dose or isocenter. The bowtie filter minimizes attenuation of x-ray intensity to isocenter and attenuates radiation significantly with radial distance beyond the center region of the bowtie, because, ideally, the isocenter corresponds to an imaging center of the subject. However, this is not always the case, e.g. when the subject is mis-centered in the scanner.
FIG. 1 illustrates a bowtie filter ideally matched to a patient. Specifically, bowtie filter 10 is aligned within an imaging beam 12 such that an x-ray profile 14 is generated by the incidence of the imaging beam 12 upon the patient 16. However, if the patient 16 is not centered with respect to the bowtie center and the corresponding isocenter, significant image degradation can occur. The degradation is dependent upon a multitude of factors, such as the size of the central region of the bowtie filter, size and shape of the patient, and the amount and direction of patient mis-centering. FIG. 2 illustrates one such example of a bowtie filter opening that is improperly matched to the patient. That is, the bowtie filter 10 is aligned within the imaging beam 12 such that an improper x-ray profile 18 is generated by the incidence of the imaging beam 12 upon the patient 16. Specifically, photon incidence or flux at the edges of the patent may increase image noise to a level that may be prohibitively high for diagnostically valuable images.
Recent improvements in imaging devices include a continuously adjustable bowtie filter having a pair of filtering elements to compensate for factors that may lead to non-ideal imaging. Such a filter is described in U.S. Ser. No. 10/605,789, the disclosure of which is incorporated herein and is assigned to GE Medical Systems Global Technology Co., LLC, which is also the Assignee of this application. Each filter element has a long low attenuating tail section that varies in attenuation power across its length such that as the elements are moved relative to one another, the attenuation of the beam is controlled. Each filter element is dynamically positioned with a dedicated motor assembly. The filter elements may be positioned in the x-ray beam so as to shape the profile of the x-ray beam to match a desired ROI or anatomical point-of-interest.
The filter portions are positionable and adjustable using precision positioners to control the radiation pattern for the patient or the anatomy currently being imaged. However, image degradation may occur if the bowtie opening created is too small for a large patient since useful x-ray needed for imaging is attenuated by the bowtie thereby causing high image noise. As a result, the operator must manually determine the appropriate beam width and position according to size, shape, and positioning of the subject within the scanner bore.
A properly sized bowtie configuration, however, does not ensure acceptable image quality. If the subject is mis-centered, image degradation may still persist. This degradation is typically a result of two factors. First, if subject mis-centering is caused by mis-elevation of the subject with respect to the bowtie filter then the calculation of tube current will result in an underestimate of the subject size. Referring to FIG. 3, a patient 16 is shown mis-centered in an x-ray beam 12. Specifically, the patient 16 is positioned at an improper centering elevation 20 by a centering error 22 below a proper centering elevation or y-position 24. As a result, a portion of the imaging beam 26 is not incident upon the patient 16 and a projection area 28 is understated by an error margin 30 because the patient 16 intercepts fewer rays in the imaging beam 12. As such, when determining tube current with the imaging tube at top-dead-center, as is convention, a lower tube current than actually required for proper imaging will be determined. As a result of the lower tube current, excessive image noise will be present relative to the user's selection. For example, a calculated milliamp (mA) that is 30% lower than actually required for proper imaging occurs for a typical 30 cm×20 cm body mis-centered in elevation by three cm. In such a case, noise introduction is increased by approximately 15% from a properly centered, properly imaged, patient.
Secondly, patient mis-centering with respect to elevation may also position the thickest part of the patient such that x-rays for lateral projections pass through the thickest part of the bowtie which results in over-attenuation of the imaging beam. Referring to FIG. 4, the patient 16 is shown mis-centered within the imaging beam 12 by a centering error 22 below the proper centering elevation 24. As a result, the imaging beam 12 passes through the thickest parts of the bowtie filter 10 and patient 16, as exemplified by projection route 32. Such mis-centering can result in an additional image noise increase by as much as 70%. These errors can cause images of such high noise that the diagnostic value is compromised. Moreover, since traditional CT imaging methods rely on operator input to perfect patient centering, including elevation, elevational patient mis-centering can be common. Furthermore, traditional edge detection methods rely on identifying the center of the patient indirectly by detecting the edges of the patient, which can be particularly susceptible to error.
Additionally, recent advancements in detector technology has increased the desire to control x-ray flux management to within very accurate constraints. For example, photon counting (PC) and energy discriminating (ED) detector CT systems have the potential to greatly increase the medical benefits of CT by differentiating materials such as a contrast agent in the blood and calcifications that may otherwise be indistinguishable in traditional CT systems. Additionally, PC and ED CT systems produce-less image; noise for the same dose than photon energy integrating detectors and hence can be more dose efficient than conventional CT systems. However, while PC and ED CT systems have the potential to realize numerous advantages over traditional CT detectors, the systems may be impractical for some scan protocols.
Therefore, it would be desirable to design an apparatus and method to automatically control flux by dynamically filtering radiation emitted toward the subject during radiographic imaging in a manner tailored to the position and/or shape of the subject to be imaged so as to optimize radiation exposure during data acquisition. It would be further desirable to have a system that tailors the radiation emitted toward the subject during data acquisition based on a scout scan of the subject. Furthermore, it would be advantageous to have a system and method of controlling x-ray flux management to avoid photon pileup. Additionally, it would be desirable to have a system and method of dynamically adjusting radiation filtering to follow a user defined-region-of interest. It would also be desirable to have an apparatus to automatically collect patient centering and surface elevation information include a direct method of detecting patient centering. Furthermore, it would be desirable to have a method of accurately determining patient mis-centering within an imaging volume and adjusting the patient position to compensate for the determined mis-centering.